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 MYM,Rameez August 10, 1999 02:34

goal of CFD

Tell me some problems or goals now CFD is faced to do

 Francisco Saldarriaga August 10, 1999 20:32

Re: goal of CFD

I take your question a one on areas of research. In turbulence LES is one of the most recent accepted models that works well in predicting flows in cases like flow and heat transfer past backward facing steps or past a surface with ribs; however it is still a somewhat new method that needs validation for many other situations. These situations are those involving near wall conditions. The science still cannot get to the wall and modelling of the small eddies is necessary. The search for a universal model continues. Apply this to all the aspects of life thinking that fluids are all around us and within us. The goals are then limitless.

 Jin Wook LEE August 13, 1999 00:40

Re: goal of CFD

Dear friends

Recent my research area is releted to the simulation of two phase(solid-gas, droplet) combustion. When, I start the simulation of new category among the wide range of combustion, I always think that 'how can I obtain the CONVERGED solution ? or 'what is the best strategy with paying less effort ?' even when I use commercial package.

As far as combustion problem was concerned, it was fairly difficult to obtain 'WELL CONVERGED SOLUTION'.

I guess that many of you had spent the entire night to obtain converged solution except when the problem was fairly simple, for example, simple fluid flow with simple geometry. Of course, it's basic problem is originated from the non-linearity of governing equation(Navier-Stokes eq.). However, if we can obtain good solution without paying much effort, for example, paying same degree of effort for LINEAR structrual analysis, we can contribute much to the engineering/design area and science area.

There might be many goal of CFD. As a personal point of view, I think that one of the most important goal of CFD is the development of algorithm which ensure the convergency, at least better than current situation.

Sincerely, Jin Wook LEE

 John C. Chien August 13, 1999 02:18

Re: goal of CFD

(1). From the begining of the computer age, to the modern days of supercomputers, CFD has been used to design the best weapon system. (2). It will remain so into the next century. (3). Relatively speaking, the use of CFD in the areas other than weapon system development is not essential. That is to say, the total befinit is likely to be negative, and expensive. In those areas, the product design can be accomplished without the use of CFD at all. This includes the design of cars and every day household products. (4). You think you will be able to design an efficient combustor ( or a reliable working combustor ) by solving chemical reactions in turbulent flows? (5). So far, CFD is an opportunity to provide an engineer or researcher to do more thinking. In the process, some new ideas and design will emerge as by-products. (6). To put a working scheme into a code and make it available to the public is unlikely to happen, yestereday, today or tomorrow. (7). So, all we can hope is through the use of non-working or non-converging CFD codes, real useful ideas will be discovered. So, in that sense, CFD is providing a good opportunity. (it even provides the CFD code vendors to make some money, the hard way I think.)

 Tapio Paattilammi August 13, 1999 10:00

Re: goal of CFD

Goal of CFD

CFD discussion in this and many other forums sometimes reminds me about missionary work. I have seen papers with comments like "CFD must better be implemented in Industry ..." etc.

Many CFD related research papers are very much dealing with motivation how CFD is beneficial for industry. Ofcourse, this motivation is sometimes necessacy. CFD may be, if properly used, very beneficial. But I still feel bit irritated by meaningless sentences: "CFD provides us detailed insight information....", "CFD can be used as an optimisation tool ......". I quess it is part of "standard style" of research paper. How many of you really have been thinking about what those sentences actually mean ? Do we really need so detailed results that are actually useless if we are not able to interpret them in order to make design improvements ? What is meant by optimization ?

Most products designed in industry have very little potential in fluid mechanical "optimisation". They are very complex units, fluid flow related phenomena are just one part of them. My personal opinion is that most fluid flow problems can be solved most efficiently by the use of simple basic thermodynamics and fluid mechanics theory: heat transfer, turbulence, boundary layer theory,... People studying and using CFD should have solid theoretical background in (analytical) fluid mechanics and thermodynamics. The use of CFD is not a shortcut to any fluid flow problem. When solving flow problems I have been happy with books like "Handbook of Hydraulic Resistance" based on huge amount of experiments. Rapid prototyping combined with advanced measuring techniques may become a serious competitor for CFD.

Design work is always a team effort. There people from marketing, manufacturing, stress analysis and so on. So, fluid flow is not everything.

"Industrial" flows are mostly extremely complex. They include laminar to turbulence transition, multi-phase flows, combustion, swirling flow, heat transfer and many other topics which are very well known to be difficult to solve by CFD.

Many of the CFD users (including me !) are often very enthusiastic about it. It is very challenging and interesting, ofcourse. But we must not forget the common sense !

Okay, this all may sound bit too skeptic but I think it is just reality. And reality is what we are living (I hope so) in. If we are not carefull CFD becomes only virtual reality.

----- Tapio

 Jin Wook LEE August 15, 1999 21:32

Re: goal of CFD

Dear John

MYM Rameez proposed very interesting topic, 'Goal of CFD'. Francisco Saldarriaga presented an opinion about LES. And my opinion was one of the possible personal point of view about convergence.

Tapio Paattilammi gave us very useful comments which, I think, most CFD researchers/engineers should be careful of. I really assent to, and respect his sincere opinion.

However, in your case, I agree some of your opinion but some comments were very discouraging to me, and perhaps to many CFD researchers/engineers who are not in weapon area.

(1) Of course, some works by CFD are cost ineffective. However, your comment, 'all works by the use of CFD in the areas other than weapon system is not essential', is seemed to be not proper for many CFD works. I think, many works are cost effective even though they are not related to weapon system. As my personal opinion, cost effectiveness is dependent of user's capability such as knowledge about CFD, insight about the system which he is going to analyze and his experience of both CFD and the system. There might be not so many engineers/researchers who have all these ability but, I think that there are many teams which have such capability.

(2) Working and converging codes are better than non-working and non-converging codes. Of course, very useful ideas can be discovered through the use of non-working and non-converging codes as you said. But, in this case, the discovery is only your (his, her) property, but useful converged solution can be shared. I think that more useful idea can be obtained from the post-processing(the deep consideration of the results, not simple graphical display) of the converged solution. As additional personal point of view, what we really should be aware of is, CFD can be very useful if and only if we know its capability and limit and when it is properly used to the possible application. I still hope that convergency of CFD in the future would become better than now because something is better than nothing.

Sincerely,

 John C. Chien August 15, 1999 23:08

Re: goal of CFD